Photometric measurements of optical transmittance of a liquid involve a probe or other device for passing a light beam through the liquid and passing the beam to a photometric system such as a spectrophotometer. Various configurations have been disclosed for such a device, sometimes called a cuvette. For example U.S. Pat. No. 4,431,307 discloses a cuvette in which radiation is passed through liquid in a container and thence to a photodetector embodied in the cuvette. Coating of side walls minimizes loss of radiation. U.S. Pat. No. 4,648,713 discloses a couvette having at least two regions of different path length through the liquid.
Recent developments in photometric instruments such as spectrophotometers have provided for greater precision, particularly with computerized operation, for example in automated comparisons of sample spectra. An example of a potential application for a new instrument is in-situ detection of near-infrared radiation transmission through gasoline, in which very small variations in transmission are associated with octane levels. A need has evolved for greater precision from the various components of the instruments. One such component is a probe or other apparatus for selectively passing a light beam through a liquid and a reference medium.
It is desirable that optical paths be nearly identical for the optical trains for the sample fluid and the reference medium. Optical transmissions should be very stable and generally immune to significant changes in the refractive index of the liquid, such as from temperature and pressure changes. The overall result should be that a ratio or difference of spectra of the sample and a standard provide a very accurate measure of the spectrum of the fluid.
Further problems exist with analysis of an inflammable fluid such as gasoline. Any electrically driven components in such apparatus, particularly the means for selectively passing the light beams, must be spark-free and preferably free of any electrical current flow. In conventional induction motors, coils generally are disposed radially of and in close proximity to a rotory magnet, and such motors may be isolated from a combustible zone by extending the axle through a seal which is subject to leaks. Coupling may also be effected by coaxial rotary magnets separated by a barrier wall to isolate the combustible zone.